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Chapter 8 — Nature of Matter: Elements, Compounds, and Mixtures 119 z Stir continuously to make a solution of calcium Lime hydroxide. This solution is called lime water. water z Filter it and observe its colour. Petri z Leave this colourless solution in a petri dish for a few dish hours (Fig. 8.5a). (a) z Keep stirring the solution at regular intervals. z What do you observe (Fig. 8.5b)? Lime water z Does it turn milky? turns Can you explain why the solution has turned milky? milky You know that lime water turns milky when carbon Petri dioxide reacts with calcium hydroxide to form calcium dish carbonate (insoluble tiny white particles) and water (b) (Fig. 8.5). Since lime water turns milky when exposed to Fig. 8.5: Lime water turns milky air, this activity demonstrates the presence of carbon in the presence of carbon dioxide dioxide in the air. Calciumhydroxide+Carbon dioxide Calciumcarbonate+Water Apart from gases, have you ever observed anything else present in the air? Have you ever noticed tiny shining particles moving in a beam of sunlight entering a dark room through a small opening? What are these particles? Activity 8.2: Let us explore z Take a black sheet of paper. Ensure that it is free from (a) any visible dust particles. z Place the black sheet of paper undisturbed near an open window (Fig. 8.6a), or in the garden, for a few hours. z What do you observe? You may notice tiny particles settled on its surface. You may use a magnifying glass to examine the particles more closely (Fig. 8.6b). This shows that dust particles are suspended in the air. (b) They are not an integral part of the air and are considered Fig. 8.6: Black sheet of paper pollutants. The nature and the number of dust particles in (a) with no dust particles; the air may vary from time to time and from place to place. (b) with dust particles A step further The major pollutants present in the air are particulate matter (dust, soot) and gases like carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide. The air quality index (AQI) is a tool used to describe the air quality. Reprint 2026-27